1. Field of the Invention
This invention relates to mineral beneficiation by the separation of a preferred component from a mixture by froth flotation. More particularly, this invention relates to an improved method and apparatus for the separation of finely ground minerals and contaminants by combining froth flotation separation with density separation techniques.
2. Description of Related Art
Flotation, in particular froth flotation, is one of the primary solid-solid separation processes for fine particles. The process has been widely practiced for almost a century in the mining industry for concentrating valuable minerals such as phosphate rock, precious metals, lead, zinc, copper, molybdenum, and tin containing ores as well as coal. Typically, the froth flotation process has been developed to work in water, with air as the froth generating gas, however, other liquid and gas combinations can be used
With the froth flotation process one or more specific particulate constituents of a slurry or suspension of finely dispersed particles become attached to gas bubbles so that they can be separated from the other constituents of the slurry or suspension. The froth flotation process exploits the wettability differences of the particles to be separated. Differences in the wettability among solid minerals particles can be natural, or can be induced by the use of chemical additives. The buoyancy of the bubble/particle aggregate, formed by the adhesion of the gas bubble to a particle in the slurry, is such that it rises to the surface of the flotation vessel where it is separated from the remaining particulate constituents which remain suspended in the aqueous phase of the suspension.
The particles to be separated by the froth flotation process are in the size range of about 500 .mu.m to 2-10 .mu.m; however, 65 mesh (230 .mu.m ) to 270 mesh (53 .mu.m) is typical. Raw ore is comminuted in size from boulders of up to 100 cm in diameter to a size range of from about 3 cm to about 0.5 cm using jaw crushers, cone crushers, gyratory crushers, or roll-type equipment. If ore of this size is to be used in a subsequent process, the sized ore is sieved and/or washed to remove impurities that concentrate in the fine particle size range. When the entire volume of ore is to be processed by froth flotation further size reduction using rod mills and ball mills is used to bring the particle size of all the ore to finer than about 65 mesh (230 .mu.m). The primary objective of this is to generate mineral grains that are discrete and distinct from one another. The generation of distinct particles is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation cell by gas bubbles that adhere to them.
The flotation step is accomplished by the preparation of pulp, consisting of a solid-liquid slurry that may contain up to 40% solids, to which chemical reagents known as collectors are added in a conditioning tank. Selected reagents are added to render some minerals hydrophobic so that they selectively adhere to air bubbles introduced into the pulp in a flotation cell. On the other hand, some reagents are added to enhance selectivity through activation and depression phenomena. Frothers are also used to generate a mineral-laden froth layer and enhance particle-bubble adhesion. The products from the flotation cell are a concentrate and a tailing stream. The concentrate proceeds to the next step for further cleaning or treatment. A typical froth floatation process can treat, for example, a raw feed that assays 0.5% to a few percent copper to give a mineral concentrate analyzing 35% copper with a recovery of more than 85% of the copper content of the original ore.
The actual flotation process occurs in flotation cells usually arranged in batteries in an industrial plant. The individual cells can be any size from a few to 30 m.sup.3 in volume. Also, column cells have become popular, particularly in the separation of very fine particles in the minerals industry and colloidal precipitates in environmental applications. Such cells can vary from 3 to 9 meters in height and have a cross section of 0.3 to 1.5 meters in width.
Traditionally, in the U.S., only about 5 percent of fine coal is cleaned by froth flotation because of technical difficulties and unfavorable economics. Fine coal processing by froth flotation is associated with difficulties in froth handling, product dewatering, low throughput, and inefficient separation of impurities such as pyrites. Therefore, traditionally, a majority of the coal in the U.S. is cleaned at coarse and intermediate sizes (down to 28 mesh) by gravity separation. A significant portion of the coal fines (minus 28 mesh or less than 0.595 mm.--equivalent to 0.0234 inches) are discarded as waste into tailings ponds. Therefore, it is believed, there is a need to augment the froth flotation process and the flotation column in particular with an efficient secondary separation process based on density separation in order to improve the utilization of fine coal.
It has been noted in the art that the froth flotation process does not always provide complete separation of desired materials from unwanted impurities. A number of modifications have been suggested to the froth flotation process to improve the efficiency of the separation.
A method and apparatus for separating coal or mineral ore fines by froth flotation are disclosed by Miller et al., U.S. Pat. No. 4,744,890. Miller discloses a countercurrent flotation device and method that use a vertically oriented, cylindrical flotation vessel having a tangential inlet at its upper end and an annular outlet at its lower end. A pedestal positioned within the lower end of the vessel serves to support the froth column formed within the flotation cell and to minimize mixing between the froth column and the fluid discharge. The configuration of the flotation vessel, with its tangential inlet and annular outlet directs the particulate suspension around the vessel in a swirling motion. The froth column, which carries one component stream, exits through the top, center of the column, while the second component exits around the outer perimeter at the bottom of the column. This design has the disadvantage that the fluid flow and the centrifugal forces are coincident, making it difficult to separate the effects of froth flotation from the secondary separation method. Further, this system utilizes a porous column wall to introduce gas into the flotation process which acts counter to density separation by the use of centrifugal force.
An alternate method and apparatus for separating coal or mineral ore fines by a swirl-flow pattern to develop centrifugal forces on the liquid or gas stream are disclosed by Duczmal et al., U.S. Pat. No. 5,224,604. Duczmal discloses an air-sparged hydrocyclone flotation device and method for the separation of particles in either liquid or gas streams. The fluid stream is directed in a swirl-flow pattern in a porous-walled cylinder to develop centrifugal forces on the stream. Magnetic or electrical fields can be applied to the system to enhance separation of the particles. Air sparging may also be employed to further amplify the separation of hydrophilic particles from hydrophobic particles in a liquid system. The swirl-flow pattern exits the downstream end of the separator where a stream splitter is employed to split the swirl-flow pattern stream which splays outwardly at the outlet in two or more streams which carry desired particles to be recovered. The hydrocyclone of Duczmal has the disadvantage that the fluid flow and the centrifugal forces are coincident making it difficult in separating the effects of froth flotation from the secondary separation technique/method. Further, the hydrocyclone of Duczmal provides minimal mixing of the materials to be separated. Also, in one embodiment, both impurities and desired products enter from the same end and are discharged at different radii from the other end.